1. Field of the Invention
This invention relates to a battery charging apparatus for series battery, which battery being mounted on electric cars and being used on robots, carriers as well as used for emergency power source.
2. Description of the Related Art
In recent years motor-driven electric cars are becoming increasingly popular for which clean and pollution-free batteries are used as a power source. Batteries mounted on such electric cars must satisfy the following requirements. Substances contained in the battery should not leak out as a result of an accident or other causes. They must be superior in terms of resistance against vibrations and shocks. They must also have a long service life even after repeated charge and discharge cycles. Various improvements have been made to satisfy these requirements and now a lead-sealed battery with excellent properties has been developed.
Moreover, each electric car incorporates a battery charging apparatus for charging batteries mounted thereon which can be recharged with a commercial A.C. power source when they are exhausted.
As a conventional battery charging apparatus designed for mounting on cars, a battery charging apparatus for a series battery consisting of battery chargers connected in series for batteries connected in series is known. For example, as shown in FIG. 6, the configuration of a battery charger C connected in series with the beginning terminal 2 and the ending terminal 3 of a column of batteries 1 consisting of 3 batteries B1, B2 and B3 is an example.
Such a battery charging apparatus for a series battery has a problem in that uneven characteristics of batteries connected in series inhibit good recharging.
For example, suppose that on an electric car a series of 12 batteries 12 V-25 Ah connected in series and a conventional battery charging apparatus for a series battery are mounted, they repeat cycles of charge and discharge until a drop in the voltage of the batteries no longer allows the car to run (for example, approximately 100 charge and discharge cycles). Suppose further that from among such batteries, a battery with a small balance of residual capacity (hereinafter referred to as battery B1), one with a medium balance of residual capacity (hereinafter referred to as battery B2) and one with a large balance of residual capacity (hereinafter referred to as battery B3) are chosen. Here, the battery with a large balance of residual capacity is a good battery whose characteristics as a battery have degenerated little, with a large capacity to discharge. The one with a small balance of residual capacity represents a battery of inferior performance whose characteristics as a battery have degenerated much, with a small capacity to discharge.
In order to compare the characteristics of these chosen batteries, the A.C. input terminals 4 and 5 of the battery charger C of the conventional battery charging apparatus are connected with the A.C. power source of 100 V, and the chosen batteries of different balances of residual capacity are connected in series to be recharged in series at a temperature of 20.degree. C. FIG. 7 shows the relationship between electric current and voltage.
In FIG. 7 in which the current flowing in the charging circuit is represented by IO, the voltage of the battery B1 with a small balance of residual capacity is represented by E1, the voltage of the battery B2 with a medium balance of residual capacity is represented by E2 and the voltage of the battery B3 with a large balance of residual capacity is represented by E3. The charging current IO, after having remained firm at 9.75 A for the first one hour of charging, gradually decreased, and one hour and 10 minutes later at the point X the first gassing took place. During the Y interval from one hour and a half later until 3 hours later, continuous gassing occurred. In the Z interval from 3 hours and 20 minutes later, up to 40 minutes later the battery B1 discharged hydrogen gas.
The charging voltage E1 of the battery B1 with a small residual balance of capacity gradually increased, surpassed the optimum voltage of 14.5 V and reached 17 V about one hour later. Then, after maintaining this level, the voltage gradually declined. After the Z interval during which hydrogen gas was fully discharged, the voltage again rose. The charging voltage E2 of the battery B2 with a medium balance of residual capacity reached the optimum charging voltage about 3 hours and 20 minutes later, keeps on rising sharply and then stabilizes at a constant level. Thereafter, voltages E1 and E2 repeat voltage crossings. The charging voltage E3 of the battery B3 with a large balance of residual capacity remains at about 13 V and ends up with an insufficient charge.
When a conventional battery charging apparatus for a series battery is used for charging batteries, therefore, the battery B1 with a small balance of residual capacity and an inferior performance develops gassing which results in a rise in its internal pressure and discharges hydrogen gas resulting in damages to itself. The battery B3 with a large balance of residual capacity and a good performance, on he other hand, ends up with an insufficient charge. In other words, the use of a conventional battery charging apparatus for a series battery leads to a situation where batteries of inferior performance affect those of good performance and uneven characteristics of batteries connected in series inhibit any good charging.
In the manufacturing process, batteries of more or less even characteristics are normally produced in each case consisting of 6 cells. Their characteristics are, however, somewhat uneven among cases. In addition, the arrangement of batteries being charged results in different radiating conditions ,and depending on their temperature, the optimum charging voltage is different for each battery.
Normally, in spite of such differences in optimum charging voltage depending on the characteristics and temperature of batteries, batteries are being charged at a same voltage. For this reason, a relatively small difference in internal resistance kept at first among batteries intensifies as charging in series is repeated. This difference in internal resistance shows up in the form of uneven characteristics of batteries, and gradually intensifies as discharges and charges are repeated.
If the charging voltage is not controlled according to the temperature of batteries, therefore, their service life is reduced. In particular when a quick charging of 5 to 10 hours is given, a heat runaway may damage the batteries in a short length of time.
Such uneven manufacturing and charging conditions produce gradual changes in the internal resistance and characteristics of batteries. According to the conventional serial charging method, the presence of a single battery with inferior performance results in insufficient charge in batteries of good performance. This condition cannot be changed by replacing a battery or batteries of inferior performance among the batteries connected in series and the presence of a battery of inferior performance among the batteries connected in series results in insufficient charging of batteries of good performance. In the actual situation, therefore, all the batteries connected in series including those having enough power are replaced.
In order to solve these problems, as shown in FIG. 8, the configuration of a plurality of batteries B1, B2 and B3 connected in series being connected in parallel with the same number of battery chargers C1, C2 and C3 to charge separately respective battery B1, B2 and B3 has been proposed in the past. The method of charging separately, however, requires two large cables 10 for each battery for wiring with batteries. For n number of batteries, 2.times.n cables are required. For this reason, the mounting of batteries in an electric car requires many large cables wired in a narrow space of the car and also much troublesome works.